1. Field of the Invention
The present invention relates to a magnetic recording medium for use in, e.g., a hard disk drive using the magnetic recording technique, and a magnetic recording/reproducing apparatus using the magnetic recording medium.
2. Description of the Related Art
To increase the recording density of a magnetic recording medium, it is important to maintain the anisotropy of a magnetic recording layer and at the same time increase the recording resolution by decreasing the size of magnetic crystal grains. To decrease the size of crystal grains in the magnetic recording layer, various underlayers and seed layers are conventionally used.
For example, Jpn. Pat. Appln. KOKAI Publication No. 2002-197635 proposes a method in which the surface of a soft magnetic layer is partially or completely oxidized, and an orientation control layer having a face-centered cubic structure (fcc) or hexagonal closest packed structure (hcp) is stacked on the oxidized surface to decrease the grain size of this orientation control layer, thereby improving the noise characteristics of a magnetic recording layer formed on it.
Also, Jpn. Pat. Appln. KOKAI Publication No. 2002-260207 proposes a method in which an orientation control layer which preferentially orients a body-centered cubic structure (bcc) nonmagnetic underlayer in the direction of (200) is formed below the nonmagnetic underlayer by oxidation or nitriding, thereby decreasing the grain size of the nonmagnetic underlayer and that of a magnetic recording layer formed on the orientation control layer.
By these methods, crystal grains of the nonmagnetic underlayer and magnetic recording layer can be grown finely to a certain degree by using the partially oxidized or nitrided material as a core. However, the degree of fineness cannot be well controlled. In addition, when the surface of the soft magnetic film is partially or completely oxidized or the orientation control film is oxidized or nitrided, the orientation of the orientation control film degrades. Therefore, although the grain size of the orientation control layer decreases, the orientation of the layer becomes unsatisfactory and worse than that of a layer not partially or completely oxidized or nitrided. As a consequence, the characteristics cannot be well improved.
Also, Jpn. Pat. Appln. KOKAI Publication No. 2002-133645 discloses a magnetic recording medium having an arrangement in which a nonmagnetic interlayer having a granular structure made up of nonmagnetic crystal grains and a metal oxide or carbide surrounding the grains and a magnetic recording layer having a granular structure made up of ferromagnetic crystal grains and a metal oxide or carbide surrounding the grains are stacked on a substrate.
Unfortunately, granular-structure grains are three-dimensionally dispersed in an orientation control layer. Therefore, if this layer is used as a magnetic recording layer, the grains are more or less separated from each other to decrease saturation magnetization Ms of the magnetic recording layer, and as a consequence the output lowers. Also, since the orientation is random, the size of the magnetic grains must be increased to a certain degree in order to obtain sufficient magnetic anisotropy equal to a high recording density. This undesirably lowers the recording density.